1. Field of the Invention
The present invention relates to an imaging device, and particularly relates to an imaging device in which the generation of smear is prevented.
2. Background Art
A basic cell of a CMOS sensor, an active type XY address-type solid imaging device is shown in FIG. 10. In FIG. 10, the reference numeral 10 denotes a solid imaging system (a CMOS sensor), 11 denotes a P-type silicon substrate, 12 a P-type well, 14 an N-type region (photoelectric conversion region) forming a photodiode, 16 a gate SiO.sub.2 film, 18 a polysilicon film forming a reset gate, 20 an N.sup.+ region for forming a reset drain, 21 a field oxide film for element separation, and 22 a metal film for forming a light shield film in which an aperture 23 is formed for defining a light admitting area.
An interlayer insulating film 24 and a plurality of wiring layers 13, 15, 17, 19 are provided between the photoelectric conversion region 14 and the light shielding film 22, and the photoelectric conversion layer 14 is connected with a source-follower-amplifier 24 by a wiring layer (not shown).
The source-follower-amplifier 24 comprises a selection switch MOD transistor 26, a MOS transistor 28 for detection, and a MOD transistor 29 as a load, and the gate of the MOS transistor 28 is connected with the photoelectric conversion region 14.
An operation of such a CMOS sensor constituted as shown above is described as follows. First, by applying a high pulse .phi..sub.R to the reset gate 18, the potential of the N-type region forming the photodiode 14 (photoelectric conversion region) is set. Next, by applying a low pulse .phi..sub.R to the reset gate 18, electric charge generated by the photoelectric conversion is stored in the depleted layer. Depending upon the amount of the stored electric charge, the potential of the N-type region (photoelectric conversion region) 14 is changed, and the potential change is output from an output terminal 30 of the source-follower-amplifier 24.
In the structure of the conventional CMOS sensor 10, since there are many components intervening in between the light shielding film 22 defining the aperture 23 and the photoelectric conversion region 14 on the silicon substrate 11 such as the interlayer insulating film 24 and a plurality of wiring layers 13, 15, 17, 19, the interval L.sub.0 between the light shielding film 22 and the photoelectric conversion region 14 becomes as large as a few .mu.m which is far larger than the wavelength of visible light (approximately 80 to 770 nm), and the diffracted light 25 diffracted by the aperture 23 is admitted to the periphery of the photoelectric conversion region 14 as shown in FIG. 10(a).
Therefore, in the structure of the conventional CMOS sensor, a problem arises that false signals (generally called "smear") are generated by the photoelectric conversion due to the expanded beam of light admitted to the periphery of the photoelectric conversion by the light diffraction effect and, thus, the S/N ratio of image signals is degraded.
Although a technique has been proposed in which the size of the aperture 23 (e.g. the width W.sub.0) is reduced relative to the size of the photoelectric conversion region 14 (e.g. the width W.sub.14), a problem has been encountered that this technique causes reduction of the amount of light incident to the photoelectric conversion region, which results in reducing the sensitivity.